An electrician or homeowner installs cables or wires through a wall or electrical conduit. To perform this task, a fish tape is typically used. The fish tape is unwound or paid out and fed through the wall or conduit. Once the end of the fish tape reaches its destination, the cables or wires are connected to the end of the fish tape. Then, the fish tape and connected wires are pulled back through the wall or conduit. This process is typically referred as a cable pull.
It is beneficial for the electrician to know the total length of the cable pull. This allows the electrician to confirm there is sufficient cable or wire length prior to starting the cable pull. Also, multiple wires could be cut from the same spool for one cable pull without wasting cable through inaccurate estimation of the total pull length. Wasted wire during a cable pull is costly. Another benefit is that the electrician can use the known length of the paid fish tape to locate an obstruction in the wall or conduit.
Often fish tape is damaged when pliers or other tools are used to aid in pulling the fish tape connected to cables and wires during a cable pull. This damage can eventually result in the fish tape breaking. The user can then reform the looped end on the remaining fish tape and continue using the fish tape. This reduces the overall length of the fish tape. Another benefit of the length measurement is to keep a record of the fish tape remaining within the case. As a result, the user does not waste time using a fish tape that does not have sufficient tape length for the cable pull.
U.S. Pat. No. 4,092,780 measures fish tape distance paid out through a mechanical device, which relies on mechanical friction for accurate readings. The measurement wheel must be in constant contact with the fish tape. As the fish tape is extended, the measurement wheel turns a mechanical display that indicates distance. This design is prone to inaccurate reading from the measurement wheel slipping during fast extension of the fish tape or contaminants on the fish tape. Also, continued frequent use will wear the measurement wheel and reduce the amount of friction available between the measurement wheel and the fish tape leading to increased amount of slipping and higher inaccuracies.
U.S. Design Pat. No. D506,939 provides dimensional marking directly on the fish tape. This allows the electrician the ability to accurately measure the pull length, however, this method requires a significant manufacturing cost to mark the tape and this method becomes more difficult for the user to identify total length if the fish tape has been broken and the laser numbers do not start from the original value.
Using an optical sensor to measure the length of a tape has been used in measurement tape devices, but not in fish tapes. For example, measuring tapes using optical sensors are provided in U.S. Pat. Nos. 4,242,574, 4,747,215 and 5,142,793. These devices are only capable of providing a distance measurement and use an optical sensor that requires a specific image or pattern to be located on the measuring tape for identifying distance paid out.